Drive circuits using two thin film transistors (TFTs), such as the circuit shown in FIG. 1 can be used to drive an organic EL device (OLED). The circuit of FIG. 1 is composed of an n-channel writing transistor T2, a holding capacitor C and an n-channel drive transistor T1. The drain of the writing transistor T2 is connected to a signal line DL, through which a signal voltage is supplied; the source thereof is connected to the gate of the drive transistor T1 and one end of the holding capacitor C; and the gate thereof is connected to a control line, through which a selection signal is supplied. Consequently, the signal voltage on the signal line DL is supplied to the one end (the gate of the transistor T1) of the holding capacitor C by raising the control line to an H level. Because the other end of the holding capacitor C is connected to a negative power source (for example, the ground potential), a voltage according to the signal voltage is held in the holding capacitor C.
The drain of the drive transistor T1 is connected to the cathode of an organic EL device OLED, the anode of which is connected to a positive power source (for example, power source potential), and the source of the drive transistor T1 is connected to the negative power source. Consequently, the drain current of the drive transistor T1 is controlled by writing a predetermined signal voltage according to a gradation to the gate node of the drive transistor T1 through the writing transistor T2, and then the drain current flows through the organic EL device OLED to make the organic EL device emit light according to the signal voltage. The transistors employed may be p channel, and circuits using p-channel TFTs are also known.
A drive circuit of the organic EL device that performs threshold value compensation of a drive transistor is shown in US Patent Publication 2004/0174349A1, for example.
For the drive transistor T1, a predetermined signal voltage according to a gradation is written in the gate node of the drive transistor T1, and the drain current thereof according to the gate voltage becomes the drive current of the organic EL device OLED. Consequently, if dispersion of the characteristics of the drive transistors T1 occurs, the display of a uniform gradation cannot be performed, and the deterioration of the quality of display such as the production of the nonuniformity of display arises. On the other hand, homogenization of the characteristics of TFTs is difficult due to the processes. For this reason, a drive circuit and drive method for compensating for the characteristic dispersion of the drive transistor T1 is desired.